1. Field of the Invention
This invention relates to an apparatus for determining the volumetric flow rate of a viscous fluid stream, such as a molten glass stream.
2. Description of the Prior Art
In the manufacture of glass fiber, it is important to know accurately the volumetric flow rate, known as the pull rate, of the molten glass emerging from the heated forming bushing. The pull rate must be varied by continually adjusting the heat input to the forming bushing to maintain the characteristics desired in the finished fiber.
An apparatus is known for determining the flow rate of a molten glass stream by detecting bubbles naturally present in the stream and measuring the time taken for a bubble to flow between two predetermined points. Such apparatus is disclosed, for example, in U.S. Pat. No. 4,517,845 to Gullfiber. This apparatus employs a pair of spaced photodetectors which generate pulses as a bubble passes, and which after suitable shaping can be used to measure the time between the passage of the bubbles past the respective detectors. From the flow velocity thus measured, the volumetric flow rate can be determined by measuring the width of the fluid stream with the aid of a linear array of photodiodes.
While the apparatus described is capable of making a rough approximation of the volumetric flow rate, it suffers from inaccuracies due to the fact that in reality the width of the fluid stream varies as it flows between the photodetectors, leading to inaccuracies in the calculated volumetric rate.
Various other systems for measuring flow rate are known and are described, for example, in the following patents:
U.S. Pat. No. 4,877,436, I. Sheinkop, discloses an arrangement where an auxiliary falling stream of molten glass passes through a laser beam and the diameter of the stream is measured along with the time taken for air bubbles or `seeds` to pass through the beam. This patent depends on the detection of specular reflection from bubbles passing through a laser beam. As it requires lots of seeds, it is not suitable for highly refined glass.
U.S. Pat. No. 4,205,973, W. H. Ryan (Owens-Illinois Inc.), discloses an arrangement whereby the rate of fall, volume and section of a gob of glass are measured by two vertically spaced photodetectors and a pair of cameras arranged at 90 degrees in a horizontal plane. This arrangement suffers from the same disadvantage as Gullfiber mentioned above.
U.S. Pat. No. 3,739,636, (AA Versaci), discloses means for injecting a bubble into a fluid conduit, measuring the time taken to pass between two optical detectors and computing the flow rate. The arrangement suffers from the same disadvantage as Gullfiber.
A number of other arrangements for measuring fluid flow rate are disclosed in U.S. Pat. No. 4,866,639, R. J. Adrian (University of Illinois); U.S. Pat. No. 4,737,178, L. G. Relfschneider et al., (Owens-Corning Fiberglas Corp.); U.S. Pat. No. 4,837,708, T. B. Wright; U.S. Pat. No. 4,331,132, S. Mukasa (Olympus Optical Co., Ltd.); U.S. Pat. No. 4,554.832 - T. Hasegawa et al. (Nippon Furnace KK Ltd.); U.S. Pat. No. 4,925,296, J. Reichmuth; GB 2083612, P. H. Casswell; DE 1934583, C. T. Boe et al. (Instituttet fur Porduktudvickling Danmarks Tekniske Hojskole); JP56-147013A, H. Mukal (Klyouto Denshi KK); JP 55-040918A, K. Yamaguchi et al, (Niksuklsou KK).
An object of the present invention is to alleviate the disadvantages of the aforementioned prior art and provide a method and apparatus capable of more accurately providing the volumetric flow rate of a stream of viscous fluid, more particularly a stream of molten glass.